Sunday, June 26, 2011

A scripting language, such as Lua, can bring huge productivity gains to a game project. Quick iterations, immediate code reloads and an in-game console with a read-eval-print-loop are invaluable tools. A less obvious benefit is that introducing a scripting language creates a clear dividing line between "engine" and "gameplay" code with a well defined API between them. This is often good for the structure of the engine, at least if you intend to use it for more than one game.

The main drawback is of course performance. It is a scary thing to discover late in a project that the game is slow because the script is doing too much. Especially since bad script performance cannot always be traced back to bugs or bad algorithms. Sure, you get those as well, but you can also get problems with "overall slowness" that cannot easily be traced back to specific bottlenecks or hot spots. There are two reasons for this. First, the slowness of script code compared to C, which means that everything just takes more time. And second, the fact that gameplay code tends to be "connection" rather than "compute" heavy which means there is less to gain from algorithmic improvements.

Part of this is a management issue. It is important to monitor the script performance (on the slowest target platform) throughout the production so that measures can be taken early if it looks like it will become a problem. But in this article I will focus on the technical aspects, specifically the C-to-Lua bindings.

It is important to note that when I am talking about performance in this article I mean performance on current generation consoles, because that is where performance problems occur. PC processors are much more powerful (especially when running virtual machines, which tend to be brutal to the cache). The extra cores on the consoles don't help much with script execution (since scripts are connection heavy, they are hard to multithread). And the PC can run LuaJIT which changes the game completely.

This may of course change in future generation consoles. If anyone from Sony or Microsoft is reading this, please add support for JITting to your next generation ventures.

Lua bindings

Apart from optimizing the Lua interpreter itself, optimizing the bindings between Lua and C is the best way of achieving a general performance improvement, since the bindings are used whenever Lua calls some function in the C code which in a typical game happens constantly.

The standard way of binding an object on the C side to Lua is to use a full userdata object. This is a heap allocated data blob with an associated metatable that can be used to store the methods of the object. This allows the user to make a call like:

game_world:get_camera():set_position(Vector3(0,0,0))

In many ways, this is the easiest and most convenient way of using objects in Lua, but it comes with several performance problems:

Any time an object is passed from C to Lua, such as the camera in get_camera()
or the vector created by Vector3(0,0,0), memory for the object must be allocated on the heap. This can be costly.

All the heap objects must be garbage collected by Lua. Calls such as get_camera() create temporary objects that must be collected at some later time. The more garbage we create, the more time we need to spend in garbage collection.

Making use of many heap allocated objects can lead to bad cache performance. When the C side wants to use an object from Lua, it must first fetch it from Lua's heap, then (in most cases) extract an object pointer from its data and look up the object in the game heap. So each time there is an extra cache miss.

The colon method call syntax world:get_camera() actually translates to something like (I've simplified this a bit, see the Lua documentation for details) world._meta_table["get_camera"](world). I.e., it creates an extra table lookup operation for every call.

We can get rid of the first two issues by caching the Lua objects. I.e. instead of creating a new Lua object every time get_camera() is called, we keep a reference to the object on the Lua side and just look it up and return it every time it is requested. But this has other disadvantages. Managing the cache can be tricky and it creates a lot more objects in the Lua heap, since the heap will now hold every object that has ever been touched by Lua. This makes garbage collection take longer and the heap can grow uncontrollably during the play of a level, depending on which objects the player interacts with. Also, this doesn't solve the issue with objects that are truly temporary, such as Vector3(0,0,0).

A better option is to use what Lua calls light userdata. A light userdata is essentially just a C pointer stored in Lua, with no additional information. It lives on the Lua stack (i.e. not the heap), does not require any memory allocations, does not participate in garbage collection and does not have an associated metatable. This addresses all our performance problems, but introduces new (not performance-related) issues:

Since the objects don't have metatables we cannot use the convenient colon syntax for calling their methods.

Light user data objects do not carry any type information, they are just raw pointers. So on the C side we have no way of telling if we have been called with an object of the right type.

Lifetime management is trickier since objects do not have destructors and are not garbage collected. How do we manage dangling pointers in Lua?

Colon syntax

With light user data we cannot use the colon syntax to look up methods. Instead we must call global functions and pass in the objects as parameters. But we can still make sure to organize our methods nicely, i.e., put all the functions that operate on World objects in a table called World. It might then look something like this:

Camera.set_position(World.get_camera(game_world), Vector3(0,0,0))

If you are used to the object oriented style this way of writing can feel awkward at first. But in my experience you get accustomed to it quite quickly. It does have some implications which are not purely syntactical though. On the plus side, this style of writing makes it easy to cache the method lookups for better performance:

This transformation is so simple that you can easily write a script that performs it on your entire code base.

The main drawback is that we are no longer doing dynamic method lookup, we are calling one specific C method. So we can't do virtual inheritance with method overrides. To me that is not a big problem because firstly, I think inheritance is vastly overrated as a design concept, and secondly, if you really need virtual calls you can always do the virtual method resolution on the C side and get the benefits while still having a static call in Lua.

Type checking

For full userdata we can check the type by looking at the metatable. The Lua library function luaL_checkudata provides this service. Since light userdata is just a raw pointer to Lua, no corresponding functionality is offered. So we need to provide the type checking ourselves. But how can we know the type of an arbitrary C pointer?

An important thing to notice is that type checking is only used for debugging. We only need to know if a function has been called with the right arguments or not. So we don't actually need to know the exact type of the pointer, we just need to know if it points to the thing we expect. And since this is only used for bug detection, it doesn't matter if we get a few false positives. And it is fine if the test takes a few cycles since we can strip it from our release builds.

Since we just need to know "is the object of this type" we can make test different for each type. So for each type, we can just pick whatever test fits that type best. Some possibilities are:

Store a known four byte type marker at the start of the object's memory. To verify the type, just dereference the pointer and check that the first four bytes match the expected marker. (This is the method I use most frequently.)

Keep a hash table of all objects of the specified type and check if it is there.

For objects that are allocated from a pool, check that the pointer lies within the range of the pool.

Object lifetimes

There are two approaches you can take to ownership of objects in the Lua interface. They can either be Lua owned and destroyed by the garbage collector or they can be owned by the C side and destroyed by explicit function calls. Both approaches have their advantages, but I usually lean towards the latter one. To me it feels more natural that Lua explicitly creates and destroys cameras with World.destroy_camera() rather than cameras just popping out of existence when the garbage collector feels they are no longer used. Also, since in our engine, Lua is an option, not a requirement, it makes more sense to have the ownership on the C side.

With this approach you have the problem that Lua can hold "dangling pointers" to C objects, which can lead to nasty bugs. (If you took the other approach, you would have the opposite problem, which is equally nasty.)

Again, for debugging purposes, we would want to do something similar to what we did with the type information. We would like to know, in debug builds, if the programmer has passed us a pointer to a dead object, so that we can display an error message rather than exhibit undefined behavior.

This is a trickier issue and I haven't found a clear cut solution, but here are some of the techniques I have used:

Clear out the marker field of the object when it is freed. That way if you attempt to use it later you will get a type error. Of course, checking this can cause an access violation if the memory has been returned to the system.

For objects that get created and destroyed a lot, such as particles or sound instances, let Lua manage them by IDs rather than by raw pointers.

Keep a hash table of all known live objects of the type.

Let Lua point to the object indirectly through a handle. Use some bits of the pointer to locate the handle and match the rest to a counter in the handle so that you can detect if the handle has been released and repurposed for something else.

Conclusions

Using light instead of full userdata does make things more inconvenient. But as we have seen, there are tricks that help overcome many of these inconveniences.

We still haven't looked at truly the temporary objects, such as Vector3(0,0,0). In my next article I will discuss what can be done about them.

Friday, June 10, 2011

Simpler programs are better programs. Today's target: strings. In this post I will show you three ways of improving your code by simplifying your strings.

1. Use UTF-8 everywhere

When I issue programming tests I always have some question about different string encodings. It is a good way of testing if a candidate can distinguish what data represents from how it is represented. But when I write code I just use UTF-8 everywhere, both in memory and on disk. Why? UTF-8 has many advantages and no serious disadvantages.

Advantages:

Using the same encoding everywhere means there is never any confusion about what encoding a certain string or file should be in. If it is not in UTF-8, then it is wrong. Period.

UTF-8 uses the standard C data types for strings: char * and char [].

ASCII strings look like ASCII strings and all functions, parsers, etc that operate on ASCII strings work on UTF-8 strings without modification.

The most common disadvantages claimed for UTF-8 are:

UTF-8 can waste memory.

Finding the i’th glyph in a UTF-8 string is expensive (O(n) rather than O(1)).

There is some truth to the first point. Yes, if your text is in Japanese, UTF-8 probably uses more memory than Shift-JIS. But I don’t think that is a major issue. First, while UTF-8 is worse than other encodings for some languages, it does pretty well on average. Second, strings aren’t a big part of a game’s memory usage anyway (if they are, you are most likely doing something wrong). And third, if you care that much about string memory usage you should probably compress your string data.

Compression will pretty much nullify any differences in memory usage caused by using different encodings, since the entropy of the underlying data is the same regardless of how it is encoded. (At least in theory, it would be interesting to see someone test it in practice.)

The second point is true but also moot, since accessing glyphs at random indices in a string is a much rarer operation than you might think. For most string operations: concatenation, parsing, etc you never have to access individual glyphs. You can just use the same implementation as you would use for an ASCII-string and it will work without modification.

In the few cases where you do need to convert to glyphs (for example for rendering) you typically do that sequentially, from the start to the end. This is still a fast operation, it is only random access of glyphs that is significantly slower with UTF-8 than with UTF-32. Another interesting thing to note is that since all continuation bytes in UTF-8 follow the pattern 10xxxxxx you can quickly find the start and end of the next or previous glyph given a char * to anywhere within a UTF-8 string.

In fact I can't think of any string operation that requires fast random access to glyphs other than completely contrived examples (given 10000 long strings, find the 1000th glyph in each). I urge my readers to try to come up with something.

2. You do not need a string class

String classes are highly overrated.

Generally speaking, code that deals with strings can be divided into two categories: code that looks at static strings (parsers, data compilers, script callbacks, etc) and code that builds dynamic strings (template formatters, debug logging, etc). In a typical game project there is a lot more of the first than the latter. Ironically, string classes don’t do a very good job with either!

For code that deals with static strings you should always use const char * rather than const string &. The former is more flexible. It allows the caller to store her strings however she likes rather than adhering to some memory model imposed by the string class. It also means that if you call the function with a static string it doesn’t get pointlessly converted to a string object.

But string classes aren’t very good for dynamic strings either, as anyone who has written something like this can attest to:

string a;
for (i = 0; i<10000; ++i)
a += "xxx";

Depending on how your string class is implemented this can be horribly inefficient, reallocating and copying the string memory for every iteration of the loop. There are various ways of addressing this: reserving memory for the string up front or using some kind of "rope" or "stringstream" class.

The simpler approach is to just use:

vector<char> a;
for (i=0; i<10000; ++i)
string::append(a, "xxx");

We represent the string as a vector of chars and provide a library of functions for performing "common string operations" on that representation.

The advantage of this over using a regular string class is that it provides a clear distinction between strings that can grow (vector<char>) and strings that can't (char *) and emphasizes what the cost of growing is (amortized linear time). Do you know the cost of growing in your string class?

3. You should almost never use strings in your runtime

The variable length nature of strings make them slow, memory consuming and unwieldy (memory for them must be allocated and freed). If you use fixed length strings you will either use even more memory or annoy the content creators because they can't make their resource names as descriptive as they would like too.

For these reasons I think that strings in the runtime should be reserved for two purposes:

User interface text

Debugging

In particular, you shouldn't use strings for object/resource/parameter names in the runtime. Instead use string hashes. This lets you use user friendly names (strings) in your tools and fast ints in your runtime. It is also a lot easier to use than enums. Enums require global cooperation to avoid collisions. String hashes just require that you hash into a large enough key space.

We hash names during our data compile stage into either 32-bit or 64-bit ints depending on the risk of collision. If it is a global object name (such as the name of a texture) we use 64-bit ints. If it is a local name (such as the name of a bone in a character) we use 32-bit ints. Hash collision is considered a compile error. (It hasn't happened yet.)

Since user interface text should always be localized, all user interface strings are managed by the localizer. The localized text is fetched from the localizer with a string lookup key, such as "menu_file_open" (hashed to a 64-bit int of course).

This only leaves debugging. We use formatted strings for informative assert messages when something goes wrong. Our profiler and monitoring tools use interned strings to identify data. Our game programmers use debug-prints to root out problems. Of course, non of this affects the end user, since the debugging strings are only used in debug builds.

Hashes can be problematic when debugging. If there is an error in the resource 0x3e728af10245bc71 it is not immediately obvious that it is the object vegetation/trees/larch_3.mesh that is at fault.

We handle this with a lookup table. When we compile our data we also create a reverse lookup table that converts from a hash value back to the original string that generated it. This table is not loaded by the runtime, but it can be accessed by our tools. So our game console, for instance, uses this table to automatically translate any hash IDs that are printed by the game.

However, recently I've started to also add small fixed-size debug strings to the resources themselves. Something like this:

As you can see, all the lookup tables etc, still use the 64-bit hash to identify the resource. But inside the resource is a 32-byte human friendly name (typically, the last 32 characters of the resource name), which is only used for debugging. This doesn't add much to the resource size (most resources are a lot bigger than 32 bytes) but it allows us to quickly identify a resource in the debugger or in a raw memory dump without having to open up a tool to convert hashes back to strings. I think the time saved by this is worth those extra bytes.